Figures released by the Health Service Executive show that over €5m has been spent on private ambulance services in the west and northwest since 2013.
The data is contained in a response to a Parliamentary Question from independent TD Denis Naughten.
He sought detail of the amount paid to private ambulance services by hospitals in the HSE's Saolta Hospital Group.
It is responsible for hospitals in Galway, Ballinasloe, Castlebar, Letterkenny, Sligo and Roscommon.
In the first five months of this year, €1.33m was spent.
A total of €2.1m was paid in 2013, while the cost of providing additional transport capacity in 2014 was €2.2m.
Figures provided by the Saolta group show that over €1.5m has been spent on private ambulances bringing patients to Mayo General Hospital in the period in question.
Letterkenny General Hospital has incurred the same costs.
Just over €1.1m has been spent by Galway University Hospitals, while it has cost €1m to pay for private ambulances to bring people to Sligo Regional Hospital.
Portiuncula Hospital has incurred charges of €366,000, with €112,000 spent for transfers to Roscommon Hospital.
Mr Naughten says these costs follow the establishment of a dedicated hospital transport service for patients two years ago.
This uses Intermediate Care Vehicles (ICV) to move patients so emergency ambulances and paramedics can respond to urgent calls.
He says the money spent on private ambulances since the beginning of 2013 would have paid for an additional 28 ICVs.
The Roscommon TD claims this would have had a dramatic impact on the delivery of the service.